f which lie
far back in the phylogenetic history of our ancestors. Hunger forms
the basis for the preservation of the individual, the sexual appetite
that for the preservation of the species, as soon as reproduction
takes place by separate sexes. All appetite belongs to the motor side
of nervous activity; there is something internal which urges us to an
act, but, on the other hand, one or more sensations may exist at the
base of this something to put it in action. I have proved, for
example, that the egg-laying instinct in the corpse fly (_Lucilia
caesar_) is only produced by the odor of putrefaction. As soon as the
antennae, which contain the organ of smell, are removed from these
flies they cease to lay, while other more severe operations, or
removal of one antenna only does not produce this result.

The mechanism of appetites is thus a lower mechanism and has its seat
in the primitive nervous centers. As _Yersin_ has proved, a cricket
deprived of its brain may copulate so long as the sensory irritations
can reach the sexual nervous centers.

We can thus say that the mechanism of appetites belongs to automatic
actions deeply inherited by phylogeny. Although complicated and
composed of coordinated reflex movements which follow one another in
regular succession, it has no actual power of modifying the so-called
voluntary acts, which depend entirely on the cerebral hemispheres, and
of which we men only have a conscious feeling. The appetites are not
capable of adapting themselves to new circumstances and cease to be
produced when the chain is interrupted. We are obliged to admit that
the instincts or appetites are accompanied by a sub-conscious
introspection which, as such, can hardly enter into direct relation
with our higher consciousness, that is, with our ordinary
consciousness in the waking state.

In spite of this, when their intensity increases, the appetites
overcoming the central nervous resistances, reach the cerebral
hemispheres, and consequently our introspection or higher
consciousness, under a _synthetic or unified appearance_, and
influence in a high degree the cerebral activities, which are
reflected in association with all the elements of what we call our
mind in the proper sense of the term, that is to say, our intellect,
sentiments and will. It is from this point of view that sexual
appetite must be considered in order to make it comprehensible. Love,
with all that appertains to it, belongs as such